Mixed silver powder and conductive paste comprising same

ABSTRACT

A mixed silver powder and a conductive paste comprising the powder are disclosed. The mixed silver powder is obtained by mixing two or more spherical silver powders having different properties from each other. The mixed powder may minimize the disadvantages of the respective types of the two or more powders and maximize the advantages thereof, thereby improving the characteristics of products. In addition, by comprehensively controlling the particle size distribution of surface-treated mixed silver powder and the particle diameter and specific gravity of primary particles, a high-density conductor pattern, a precise line pattern, and the suppression of aggregation over time can be simultaneously achieved.

TECHNICAL FIELD

The present invention relates to mixed silver powders and a conductivepaste comprising the same. More specifically, the present invention aimsto provide mixed silver powders capable of improving the physicalproperties of a conductive paste used to form electrodes for electronicproducts such as solar cells.

BACKGROUND ART

A conductive paste used for the formation of a circuit in electroniccomponents such as solar cells and touch panels is generally prepared byadding silver powder and glass frit to an organic vehicle, followed bykneading them. The silver powder used for this purpose is required tohave a relatively small particle size, a narrow particle sizedistribution, and high dispersibility for the miniaturization ofelectronic components, the high densification of conductor patterns, theprecision of line formation, and the like. In addition, the silverpowder may coagulate into flaky particles (flakes) having a size ofseveral millimeters when kneaded with a roll mill or the like in theprocess of preparing a paste. Thus, it is necessary to have a viscosityto be readily kneaded and good dispersibility in a solvent.

If the particle size characteristics of silver powder are poor, thethickness of the wires or the electrodes is not uniform, and they arenot uniformly sintered, so that the resistance of the conductive filmincreases or the strength decreases. Thus, a technique for controllingthe particle size, shape, and the like of silver powder is required inorder to produce silver powder having a shape close to a sphere withexcellent particle size characteristics without containing coagulatedparticles or coarse particles. To this end, techniques are known forcontrolling the cumulative 10%, 50%, 90%, and 100% particle sizes(hereinafter referred to as D₁₀, D₅₀, D₉₀, and D_(max), respectively) inthe particle size distribution of silver powder and the average particlesize of primary particles thereof (hereinafter referred to as D_(SEM)).

Further, if the dispersibility of silver powder in a paste is not good,the quality of the coating film formed therefrom and the linear shape ofthe line pattern deteriorates. In addition, since the fillability of thesilver powder is lowered due to the voids caused by the generation ofcoagulated particles or coarse particles, there is a problem inconductivity. Thus, attempts have been made to solve these problems.

Japanese Laid-open Patent Publication No. 2005-48237 discloses a methodof preparing a spherical silver powder having a D_(SEM) of 0.6 μm orless and high dispersibility in which an alkali or a complexing agent isadded to an aqueous solution containing a silver salt to prepare anaqueous solution containing a silver complex, and a polyhydric phenolsuch as hydroquinone is added for reduction precipitation.

Japanese Laid-open Patent Publication No. 2010-70793 discloses a finesilver powder advantageous for precise line formation in which thesilver powder has a D₅₀ of 0.1 to 1 μm, a D₅₀/D_(SEM) of 1.3 or less,and a (D₉₀−D₁₀)/D₅₀ value of 0.8 or less.

Japanese Patent No. 5505535 discloses a silver powder having a viscosityto be readily kneaded and to suppress coagulation in which the silverpowder has a ratio (SA_(B)/SA_(S)) between the specific surface area(SA_(S)) calculated from D_(SEM) and the specific surface area (SA_(B))measured by a BET method of 0.5 to 0.9 and a D₅₀/D_(SEM) value of 1.5 to5.0.

Japanese Laid-open Patent Publication No. 2013-108120 discloses a silverpowder whose particle surface is treated with a surface treatment agentin which the silver powder has a D₅₀ of 0.5 to 2.0 μm and a standarddeviation of the particle size distribution of 0.3 to 1.0 μm.

Japanese Laid-open Patent Publication No. 2017-101268 discloses a silverpowder having a surface treatment agent on its particle surface in whichthe silver powder has a D₅₀ of 0.1 to 2.0 μm, a (D₉₀−D₁₀)/D₅₀ value of2.0 or less, and a D_(max) of 5.0 μm or less and less than 7 times ofthe D₅₀, and the average value of sphericity is 1.0 to 1.5.

Japanese Laid-open Patent Publication No. 2018-80402 discloses adispersion liquid containing a silver powder and a solvent in which thesilver powder has a D_(SEM) of 0.15 to 0.5 μm and a D₅₀/D_(SEM) value of1.7 or more, and the main component of the solvent is an organiccompound having 6 to 20 carbon atoms.

However, these prior art documents only disclose the particle sizedistribution of the silver powder. They fail to recognize thecoagulation over time after the production of the silver powder and thedispersibility and sedimentation in the paste, to pay attention to theaverage particle diameter (D_(SEM)) and coagulation (D₅₀/D_(SEM)) of theprimary particles, or to take into consideration the surface treatmentof the silver powder at all. Thus, there is a limit to the densificationor fine-tuning of the pattern or preventing deterioration over time.

DISCLOSURE OF INVENTION Technical Problem

The conventional spherical silver powders have advantages anddisadvantages according to their particle size distributioncharacteristics. For example, if the particle size is relatively small,the true specific gravity is high, which raises the sinteringtemperature, resulting in an increase in resistance at low-temperaturesintering. On the other hand, if the particle size is relatively large,there is a disadvantage that clogging of printing occurs when screenprinting is carried out in a fine pattern. In particular, theconventional silver powders need to be further improved indispersibility when applied to pastes.

Accordingly, as a result of the research conducted by the presentinventors, it has been discovered that two or more types of silverpowders having different characteristics are appropriately mixed andsurface treated, thereby compensating for the disadvantages of each typeof the silver powders and enhancing dispersibility. In addition, it hasbeen discovered that if the particle size distribution of the mixedsilver powders, and the particle size and the specific gravity of theprimary particles are comprehensively controlled and the dispersibilityis increased by the combination of a surface treatment agent, it ispossible to simultaneously achieve a highly densified conductor pattern,a precise line pattern, and suppression of coagulation over time.

Accordingly, an object of the present invention is to provide mixedsilver powders, a conductive paste and a solar cell comprising the same,which are advantageous for the densification of a conductor pattern,precision of line formation, and suppression of coagulation over time.

Solution to Problem

Accordingly, the present invention provides mixed silver powders forsolving the above problems, wherein the mixed silver powders comprisetwo or more types of surface treatment agents on their surfaces.Specifically, the mixed silver powders comprise two or more types ofspherical silver powders having different particle size distributions,and the spherical silver powders comprise two or more types of surfacetreatment agents on their surfaces.

According to an embodiment, when the cumulative 10%, 50%, and 90%particle sizes by volume in the particle size distribution of the mixedsilver powders obtained by laser diffraction are referred to as D₁₀,D₅₀, and D₉₀, respectively, and the average particle size of the primaryparticles obtained by image analysis of a scanning electron microscopeis referred to as D_(SEM), the D₅₀ is 0.5 to 2.5 μm, the D₅₀/D_(SEM) is1.0 to 1.5, the (D₉₀−D₁₀)/D₅₀ is 1.0 to 2.0, and the true specificgravity is 9.4 to 10.4. According to another embodiment, the mixedsilver powders have a weight increase peak at a temperature of 200° C.to 300° C. in thermogravimetric analysis (TGA) under an elevatedtemperature condition of 10° C./min.

According to another object of the present invention, a conductive pastecomprising the mixed silver powders is provided. According to stillanother object of the present invention, a solar cell comprising anelectrode formed from the conductive paste is provided.

Advantageous Effects of Invention

According to the present invention, two or more types of sphericalsilver powders having different characteristics are mixed and surfacetreated, whereby it is possible to enhance the dispersibility while thedisadvantages of each type of the powders are minimized and theadvantages thereof are maximized. In addition, according to the presentinvention, the particle size distribution of the mixed silver powdersand the particle size and the specific gravity of the primary particlesare comprehensively controlled, so that it is possible to simultaneouslyachieve a highly densified conductor pattern, a precise line pattern,and suppression of coagulation over time.

Accordingly, the mixed silver powders are applied to a conductive pasteto increase the dispersibility and fluidity to enhance the conductivity,thereby keeping the electrode resistance low, maximizing the batteryefficiency, and securing the long-term product reliability. In addition,the process for preparing the mixed silver powders is simple, and theyhave excellent processability when applied to a conductive paste. Thus,it is possible to enhance the production efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an electron microscope image of a cross-section of silverpowder A3 prepared in Preparation Example 3 cut with a focused ion beam(FIB).

FIG. 2 is an electron microscope image of a cross-section of silverpowder B2 prepared in Preparation Example 5 cut with an FIB.

FIG. 3 is an electron microscope image for measuring the area of poresobtained by image processing of a cross-section of silver powder B2prepared in Preparation Example 5 cut with an FIB.

FIG. 4 shows the result of gas chromatography-mass spectrometry (GC-MS)of silver powder C2 prepared in Example 2.

FIG. 5 shows the result of GC-MS of silver powder C5 prepared in Example5.

FIG. 6 shows the result of simultaneous DSC and TGA (SDT) of silverpowder C5 prepared in Example 5.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is not limited to what is disclosed below. Rather,it may be modified in various forms as long as the gist of the inventionis not altered.

In this specification, when a part is referred to as “comprising” anelement, it is to be understood that the part may comprise otherelements as well, unless otherwise indicated.

In addition, all numbers and expression related to the quantities ofcomponents, reaction conditions, and the like used herein are to beunderstood as being modified by the term “about,” unless otherwiseindicated.

[Mixed Silver Powders]

The mixed silver powders of the present invention are prepared by mixingsilver powders having different characters, and they comprise two ormore types of surface treatment agents on their surfaces. Specifically,the mixed silver powders comprise two or more types of spherical silverpowders having different particle size distributions, and the sphericalsilver powders comprise two or more types of surface treatment agents ontheir surfaces.

According to an embodiment, when the cumulative 10%, 50%, and 90%particle sizes by volume in the particle size distribution of the mixedsilver powders obtained by laser diffraction are referred to as D₁₀,D₅₀, and D₉₀, respectively, and the average particle size of the primaryparticles obtained by image analysis of a scanning electron microscopeis referred to as D_(SEM), the D₅₀ is 0.5 to 2.5 μm, the D₅₀/D_(SEM) is1.0 to 1.5, the (D₉₀−D₁₀)/D₅₀ is 1.0 to 2.0, and the true specificgravity is 9.4 to 10.4.

According to another embodiment, the mixed silver powders also have aweight increase peak at a temperature of 200° C. to 300° C. inthermogravimetric analysis (TGA) under an elevated temperature conditionof 10° C./min.

Particle Size Distribution

The particle size of the mixed silver powders may be measured by a laserdiffraction particle size distribution measurement method, for example,a wet laser diffraction type particle size distribution measurement.

More specifically, 0.3 g of the mixed silver powders is added to 30 mlof isopropyl alcohol and treated with an ultrasonic washer at a power of45 W for 5 minutes to prepare a dispersion, and then the particle sizedistribution of the mixed silver powders in the dispersion is measuredwith a particle size distribution meter (e.g., Microtrac™ MT3300EXII ofNikkiso, Analysette22 of Fritsch, and the like). The cumulative 10%,50%, and 90% particle sizes by volume in the particle size distributionthus obtained are referred to as D₁₀, D₅₀, and D₉₀, respectively.

The D₅₀ of the mixed silver powders is 0.5 to 2.5 μm. Within the aboverange, a precise line can be formed, the silver powders are notexcessively activated, so that they can be sintered at 400° C. orhigher, and the linear shape of the line pattern may be excellent. If itis less than the above range, when a wiring layer is formed, theresistance of the conductive film increases, so that the conductivitymay decrease. If it exceeds the above range, the dispersibility of themixed silver powders is lowered, so that the particles coagulate duringkneading, which deteriorates the printability. More specifically, theD₅₀ of the mixed silver powders may be 1.0 to 2.0 μm.

D₁₀, D₅₀, and D₉₀ of the mixed silver powders are particle size valuesmeasured inclusive of coagulated silver particles (i.e., secondaryparticles), whereas D_(SEM) is an average particle size value of primaryparticles. Thus, as D₅₀ is closer to the value of D_(SEM), which is anaverage particle size of the primary particles, the coagulation betweenthe primary particles is less, and the particles are uniformlydispersed. In theory, D₅₀ cannot be less than D_(SEM). Thus, if thesampling error is not considered, the lower limit of the D₅₀/D_(SEM) isabout 1. For example, the D₅₀/D_(SEM) value may be 1.0 to 1.5, 1.0 to1.3, 1.0 to 1.2, or 1.0 to 1.1. Within the above range, thedispersibility is improved, which is more advantageous for the formationof precise lines.

In addition, the (D₉₀−D₁₀)/D₅₀ value of the mixed silver powders may be1.0 to 2.0 or 1.0 to 1.7. Within the above range, the mixed silverpowders have a narrow particle size distribution, and it is easier toachieve the densification of patterns, so that the linear shape of theline pattern may be excellent.

In addition, the D₁₀/D₅₀ value of the mixed silver powders may be 0.5 orless, for example, 0.1 to 0.5. Within the above range, the silverparticles having a relatively small particle size are filled between thesilver particles having an average particle diameter, so that theconcentration of silver in the conductive paste can be further enhanced.

Specific Gravity/Density

The mixed silver powders may have a true specific gravity of 9.4 to10.4, more specifically, 9.8 to 10.2. The true specific gravity may bemeasured using a true specific gravity measuring device commonly used inthe art. For example, it may be measured using AccupycII ofMicromeritics.

The mixed silver powders may have a tap density of 2.0 to 7.5 g/cc.Within the above range, the fillability of the silver powders in thecoating film is excellent, the resistance of the conductive film can bekept low, and it is more advantageous in terms of forming a highlydensified pattern. If the tap density is less than the above range, thefilling rate may be lowered, so that the resistance of the conductivefilm may be excessively high. If it exceeds the above range, thedispersibility of the silver powders is lowered, so that the particlescoagulate during kneading, which deteriorates the printability. Morespecifically, the tap density of the mixed silver powders may be 3.5 to6.0 g/cc, 4.0 to 5.8 g/cc, 4.2 to 5.55 g/cc, 4.2 to 5.2 g/cc, or 4.3 to5.1 g/cc.

The tap density may be measured, for example, by charging 20 g of themixed silver powders to a 20-ml mass cylinder made of tempered glass andmeasuring the bulk density of the mixed silver powders using a shakespecific gravity meter (KRS-409, Kuramochi Scientific InstrumentsManufacturing Co., Ltd.) under the conditions of a shaking width of 2 cmand a tap count of 500 times. As another example, the tap density may bemeasured by charging 15 g of the mixed silver powders to a 20-ml testtube using Autotap of Quantachrome, repeating 2,000 times with a 20 mmdrop, and measuring the volume.

As an example, the mixed silver powders may have a specific surface areaof 0.1 to 2.0 m²/g and a tap density of 2.0 to 7.5 g/cc.

Specific Surface Area/Sphericity

The mixed silver powders may have a specific surface area (BET) of 0.1to 2.0 m²/g. Within the above range, since the size and viscosity of thesilver powders are appropriate, it is not necessary to dilute them whenapplied to a paste, and since the concentration of silver in the pastecan be kept high, it is possible to prevent disconnection of the wires.If it is less than the above range, the silver powders coagulate toproduce coagulated silver particles during kneading, which deterioratesthe printability. If it exceeds the above range, the resistance of theconductive film may be high. More specifically, the specific surfacearea (BET) of the mixed silver powders may be 0.2 to 0.7 m²/g or 0.3 to0.6 m²/g.

The specific surface area may be measured by the BET method by nitrogenadsorption. For example, a specific surface area measuring device(Macsorb HM (model 1210) of MOUNTECH, Belsorp-miniII of MicrotracBEL, orthe like) generally used in the art may be used. The degassingconditions for measuring the specific surface area may be 10 minutes at60° C.

The mixed silver powders may have an average sphericity of 1.5 or less,for example, 1.1 to 1.5. Within the above range, the particle fillingrate may be maintained at a high level even if the spherical shape ofthe silver particles is somewhat deformed.

In addition, the mixed silver powders may have a maximum sphericity of2.0 or less, for example, 1.1 to 2.0. Within the above range, a densecoating film with a line pattern in an excellent shape is formed withoutdisconnection of the wires, resulting in an excellent currentefficiency.

Since the mixed silver powders are composed of spherical particles asdescribed above, they are well applied to photolithography, offset,dipping, and printing methods.

Silver Powders A and B

The mixed silver powders according to the present invention comprise twoor more types of spherical silver powders having different particle sizedistributions. For example, the mixed silver powders according to thepresent invention may be a mixture of 2 to 5 types of spherical silverpowders having different particle size distributions.

For example, the mixed silver powders may comprise silver powder A; andsilver powder B having a particle size distribution different from thatof the silver powder A. In such event, the silver powder A may have aD₅₀ of 0.5 to 2.5 μm, a D₅₀/D_(SEM) of 1.0 to 1.2, and a (D₉₀−D₁₀)/D₅₀of 0.9 to 1.2. In addition, the silver powder B may have a D₅₀ of 0.7 to3.5 μm, a D₅₀/D_(SEM) of 1.0 to 1.5, and a (D₉₀−D₁₀)/D₅₀ of 1.0 to 2.0.

In addition, the two or more types of silver powders having differentparticle size distributions may have different true specific gravities.For example, the mixed silver powders may be a mixture of sphericalsilver powder A having a true specific gravity of 10.0 to 10.4 andspherical silver powder B having a true specific gravity of 9.3 to 10.0.

Specifically, the mixed silver powders may comprise silver powder A; andsilver powder B having a particle size distribution different from thatof the silver powder A, wherein the silver powder A may have a D₅₀ of0.5 to 2.5 μm, a D₅₀/D_(SEM) of 1.0 to 1.2, a (D₉₀−D₁₀)/D₅₀ of 0.9 to1.2, and a true specific gravity of 10.0 to 10.4, and the silver powderB may have a D₅₀ of 0.7 to 3.5 μm, a D₅₀/D_(SEM) of 1.0 to 1.5, a(D₉₀−D₁₀)/D₅₀ of 1.0 to 2.0, and a true specific gravity of 9.3 to 10.0.

When two types of powders having different properties are mixed asdescribed above, the characteristics of the product can be improved byminimizing the disadvantages of each powder and maximizing theadvantages thereof. For example, if the silver particle A is used alone,the true specific gravity is high, which raises the sinteringtemperature, resulting in an increase in resistance at low-temperaturesintering. In addition, if the silver particle B is used alone, the(D₉₀−D₁₀)/D₅₀ is larger than that of the silver powder A, so that theremay be a problem that clogging of printing occurs when screen printingis carried out in a fine pattern. However, if these two types of silverpowders are mixed, the disadvantages can be compensated to enhance theproduct characteristics. In particular, this effect can be maximized byadjusting the mixing ratio.

Since the silver powder A has a higher true specific gravity than thatof the silver powder B, the number or size of closed pores presenttherein may be small (see FIGS. 1 and 2). For example, the silver powderA may have a porosity of 0.1 to 3%, and the silver powder B may have aporosity of 5 to 20%. The porosity may be calculated as a percentage ofthe pore area to the particle area in a cross-sectional image of silverpowder (see FIG. 3).

The mixed silver powders may comprise the silver powder A and the silverpowder B in a weight ratio of 90:10 to 10:90. Alternatively, the mixedsilver powders may comprise the silver powder A and the silver powder Bin a weight ratio of 80:20 to 50:50. Within the above range, thesinterability is increased to further reduce the resistance, and thedispersibility is improved, so that it is possible to further suppressdisconnection even in continuous printing.

Surface Treatment Agent

The mixed silver powders comprise two or more types of surface treatmentagents on the surfaces of the spherical silver powders.

That is, the individual spherical silver particles constituting themixed silver powders comprise two or more types of surface treatmentagents on the surfaces thereof.

The content of the surface treatment agents may be 3.0% by weight orless, 1.0% by weight or less, or 0.05 to 1.0% by weight, or 0.05 to 0.5%by weight, based on the weight of the silver particles. Within the aboverange, it is possible to further enhance the dispersibility when a pasteis prepared and to adjust the rheology of the paste to enhance theprintability. In addition, it is possible to further reduce the carbonremaining upon sintering the printed paste.

Examples of such surface treatment agents include fatty acids,surfactants, organometallic compounds, chelating agents, and polymerdispersants. Salts or derivatives thereof can also be used.

Fatty acids among the above have at least one carboxyl group in thechain-type hydrocarbon backbone, so that the adsorption and desorptioncharacteristics of silver powders can be appropriately adjusted. Thus,the surface treatment agent may be a fatty acid, a derivative thereof,or a salt thereof.

The carbon number of the fatty acid may be 12 to 20, more specifically16 to 18. If the carbon number of the fatty acid is within the aboverange, the dispersibility can be further enhanced without coagulation byvirtue of steric hindrance, and it is more advantageous from theviewpoint of conductivity since it is easily decomposed by sintering.

Specific examples of the fatty acid include palmitic acid, aleuriticacid, ricinoleic acid, oleic acid, and stearic acid.

Specifically, the surface treatment agent may comprise two or more typesselected from the group consisting of a C16 fatty acid having 16 carbonatoms, a C18 fatty acid having 18 carbon atoms, a derivative thereof,and a salt thereof.

More specifically, the surface treatment agent may comprise the C16fatty acid and the C18 fatty acid at a weight ratio of 20:80 to 80:20.Within the above range, it is more advantageous from the viewpoint ofdispersibility.

In addition, it is preferable from the viewpoint of hydrophilicity thatthe fatty acid contains at least one hydroxyl group in the molecule. Forexample, fatty acids in which one hydrogen is substituted with ahydroxyl group, such as ricinoleic acid and 12-hydroxystearic acid, areadvantageous from the viewpoint of hydrophilicity. If a plurality ofhydrogens are substituted with a hydroxyl group as in aleuritic acid, itmay also be advantageous from the viewpoint of thixotropy.

These fatty acids may be used alone or in combination of two or more.

A Fourier transform infrared spectrometer (FT-IR), an automatic carbonanalyzer, and a gas chromatography mass spectrometer (GC-MS) can be usedto analyze the fatty acids attached to the surfaces of the mixed silverpowders. In such event, the surface treatment agent may be detached byheating the surfaces of the mixed silver powders with a thermaldecomposer or the like or extracted with a solvent to be analyzed.Meanwhile, since fatty acids having a hydroxyl group, such as ricinoleicacid, have high polarity, the sensitivity is too low to be measured bythe above method. Thus, the functional group thereof may be methylatedto be analyzed.

As a specific example, 1 ml of a mixed solution of hydrochloric acid andmethanol (a hydrochloric acid-methanol reagent from Tokyo ChemicalIndustry Co., Ltd.) is added to 0.5 g of the mixed silver powders, whichis heated at 50° C. for 30 minutes to detach the organic substances fromthe surfaces of the mixed silver powders and to methylate the functionalgroups. After it is cooled, 1 ml of pure water and 2 ml of n-hexane areadded thereto and stirred to extract the methylated organic substancesinto hexane. The substances in hexane are analyzed using a gaschromatography mass spectrometer (GC-MS) to analyze the fatty acids onthe surfaces of the mixed silver powders.

Thermal Characteristics

The thermal characteristics of the mixed silver powders according to thepresent invention may be confirmed in a process in which organicsubstances (i.e., surface treatment agents) are detached from thesurfaces of the silver powders under an elevated temperature conditionby thermogravimetric analysis (TGA), differential scanning calorimetry(DSC), and the like.

When the mixed silver powders are heated under an elevated temperaturecondition, volatile components such as moisture present on the surfacesare separated, so that the weight is gradually reduced. When a certaintemperature is reached, the surface treatment agent is separated, andsurface oxidation of the silver powders proceeds to increase the weight.In addition, as the separated surface treatment agent burns, anexothermic peak is generated, and sintering of the silver powders mayoccur at the same time.

Specifically, the mixed silver powders have a weight increase peak at atemperature of 200° C. to 300° C. in thermogravimetric analysis (TGA)under an elevated temperature condition of 10° C./min. Morespecifically, the mixed silver powders have a weight increase peak at atemperature of 225° C. to 250° C. in thermogravimetric analysis (TGA)under an elevated temperature condition of 10° C./min. In such event,the extent of increase in weight may be 0.01 to 2.0% by weight based onthe initial weight.

In addition, the mixed silver powders may have an exothermic peak in thetemperature range of 200° C. to 300° C., or 200° C. to 250° C., indifferential scanning calorimetry (DSC) under an elevated temperaturecondition of 10° C./min. More specifically, the mixed silver powders mayhave an exothermic peak in the temperature range of 225° C. to 250° C.in differential scanning calorimetry (DSC) under an elevated temperaturecondition of 10° C./min.

The extent of increase in weight and the temperature and size of theexothermic peak may vary with the type of the reducing agent and thetype and amount of the surface treatment agent used in the preparationof the mixed silver powders. The mixed silver powders of the presentinvention have an exothermic peak at a low temperature, so thatsintering can take place quickly even at low temperatures. When they areapplied to a conductive paste for a front electrode of a solar cell, thesintering characteristics are enhanced, so that the photovoltaicconversion efficiency of the solar cell can be increased.

[Process for Preparing Mixed Silver Powders]

The mixed silver powders according to the present invention may beprepared by a process, which comprises the preparation of a silver iondispersion liquid, reduction precipitation, recovery, drying, washing,pulverization, classification, mixing, and surface treatment.

Hereinafter, each step of the preparation process will be described indetail.

Preparation of a Dispersion Liquid of Silver Ions

First, a dispersion liquid of silver ions for producing silver particlesthat constitute the silver powder is prepared. For example, as anaqueous dispersion liquid containing silver ions, an aqueous solution orslurry that contains silver nitrate, a silver complex, or a silverintermediate may be used.

The aqueous solution that contains a silver complex among the above maybe prepared by adding ammonia water or an ammonium salt to an aqueoussilver nitrate solution or a silver oxide suspension. Specifically, ifan aqueous solution of a silver ammine complex obtained by addingammonia water to an aqueous silver nitrate solution is used, it may bemore advantageous for preparing silver powder having an appropriateparticle diameter and spherical shape.

Since the coordination number of ammonia in the silver ammine complex is2, it is possible to add 2 moles or more of ammonia per mole of silver.In consideration of the reactivity, 8 moles or less of ammonia per moleof silver may be added. However, if the content of the reducing agent inthe subsequent reduction step is increased, it may have no impact onobtaining an appropriate particle size and spherical silver powder evenif the amount of ammonia added exceeds 8 moles.

In addition, a pH adjusting agent may be further added to the aqueousdispersion containing silver ions. The conventional acids or bases suchas nitric acid and sodium hydroxide may be used as the pH adjustingagent.

Reduction Precipitation

A reducing agent is added to the dispersion liquid of silver ions toreduce the silver ions, thereby precipitating silver powder.

Examples of the reducing agent include ascorbic acid, sulfite,alkanolamine, hydrogen peroxide, formic acid, ammonium formate, sodiumformate, glyoxal, tartaric acid, sodium hypophosphite, sodiumborohydride, hydroquinone, hydrazine, pyrogallol, glucose, gallic acid,formalin, sodium sulfite anhydride, and sodium sulfoxylate (rongalite).Specifically, it may be more advantageous for controlling the particlesize of the silver powder to use one or more selected from the groupconsisting of formalin, hydrazine, and sodium borohydride as thereducing agent.

The reducing agent is preferably used in an amount of 1 equivalent ormore relative to silver from the viewpoint of the yield of the silverpowder to be precipitated. If a reducing agent having a relatively lowreducing power is used, it is preferable to increase the amount of thereducing agent to be used. For example, the reducing agent may be usedin an amount of 2 equivalents or more, 2 to 20 equivalents, 5 to 20equivalents, or 10 to 20 equivalents, relative to silver.

Recovery

Thereafter, the silver powder thus precipitated by reduction isrecovered.

The recovery may be carried out by a conventional process (e.g.,decantation or the like) used in the art, and it is not particularlylimited.

In addition, it is preferable to wash the silver powder obtained byreduction as described above since they contain impurities. The recoveryand washing may be carried out separately or simultaneously, and theymay be repeated several times.

For the washing, pure water or the like may be used. An electricalconductivity of water after washing may be measured to determine thepoint of termination.

Drying

A cake containing a lot of moisture is obtained through the recovery andwashing, so that it is necessary to remove the moisture to be used assilver powder.

Thus, the silver powder recovered and washed is dried.

The drying may be carried out using reduced pressures to vacuum, warmair, dry wind, volatile solvents, compressed air, centrifugal force, orthe like. Specifically, it may be carried out using warm air underreduced pressures. In addition, the drying may be carried out at atemperature of about 100° C. or lower at which the silver powder is notsintered.

Pulverization

If physical force is applied to the aggregates of silver powders in thedrying step to remove moisture, the silver powder does not agglomeratetogether or harden, so that a pulverization step is not necessary.However, if no physical force is applied as in vacuum drying, theaggregates of silver powder from which moisture has been removed maycoagulate in the form of a lump.

Thus, the aggregates of dried silver powder are preferably pulverized.The drying and pulverization may be simultaneously carried out. Inaddition, the pulverization step is also necessary to increase theefficiency of the subsequent classification step.

The method for pulverization is not particularly limited as long as itcan pulverize the aggregates of silver powder. For example, a high-speedstirring may be used.

Classification

The silver powder thus pulverized may be classified using a classifieror the like.

In such event, if a surface treatment agent such as a dispersant ispresent on the surface of the silver powder, the surface is activatedthereby, and they are prone to adhere to the inner wall of theclassifier. In the conventional method in which the classifier itselfrotates for classification, the shape of the silver particle may bealtered by collision or coalescence of the silver particles onto theinner wall of the classifier or by collision or coalescence of thesilver particles with each other. Thus, even if spherical silverparticles are obtained in the recovery step, it may be difficult toobtain spherical silver powder upon the classification step since thesilver particles may be distorted. Accordingly, in order to preventthis, it is desirable to use a method in which the classifier itselfdoes not rotate. For example, if the silver powder is classified bygenerating a swirling flow (i.e., free vortex) by air in the classifier,the silver particles can more advantageously maintain their sphericalshape.

Mixing

Thereafter, two or more types of the spherical silver powders thusprepared are mixed. Specifically, two or more types of spherical silverpowders having different particle size distributions are mixed.

The mixing may be carried out using an apparatus capable of physicallycarrying out a mixing step. Examples of such mixing apparatus include aplanetary mixer, a concrete mixer, a tumbler mixer, a Henschel mixer,and an intensive mixer. Other apparatuses capable of mixing two or moretypes of materials than the above are not particularly limited.

Surface Treatment

A surface treatment agent is added to the mixed silver powders, so thatthe surface treatment agent is adsorbed on the surfaces of the silverparticles.

The type and amount of the surface treatment agent used for this purposeare as described above.

In addition, a surface treatment agent may be further added in theprevious mixing step to be adsorbed simultaneously with the mixing ofthe silver powders.

In addition, in order to have a solid surface treating agent adsorbed,it is advantageously liquefied to treat all the particles homogeneously.Thus, it is desirable to dissolve the surface treatment agent in alow-boiling point organic solvent or to coat it at a temperature higherthan the melting point of the surface treatment agent.

Examples of the apparatus that can be used to have the surface treatmentagent absorbed include a planetary mixer, a Henschel mixer, and anintensive mixer. It may be used with a device capable of controlling thetemperature and an apparatus capable of continuously mixing the powders.

In addition, well-known steps may be appropriately adopted besides thecharacteristic steps described above. As a result, mixed silver powderscan be finally obtained.

[Conductive Paste and Solar Cell]

The present invention provides a conductive paste comprising the mixedsilver powders.

Further, the present invention provides a solar cell comprising anelectrode formed from the conductive paste.

The conductive paste may comprise 80 to 95% by weight or 85 to 92% byweight of the mixed silver powders; 0.1 to 10% by weight, 0.5 to 3.0% byweight, or 1.0 to 2.5% by weight of glass frit; and 4.5 to 19.5% byweight or 5.0 to 14.0% by weight of an organic vehicle. As a specificexample, the conductive paste may comprise 80 to 95% by weight of themixed silver powders; 0.5 to 3.0% by weight of glass frit; and 4.5 to19.5% by weight of an organic vehicle.

Mixed Silver Powders

The mixed silver powders contained in the conductive paste may be themixed silver powders as described above. The mixed silver powders areexcellent in particle size characteristics and dispersibility, so thatthe thickness of the wires or the electrodes is uniform and that theyare readily sintered. Thus, the resistance of the conductive film can bemaintained to be low.

Glass Frit

The glass frit is a component for adhering the silver powders to thesubstrate upon sintering. It may be appropriately selected and usedaccording to the purpose.

Examples of the glass frit include bismuth silicates, alkali metalborosilicates, alkali earth metal borosilicates, zinc borosilicates,lead borates, lead borosilicates, lead silicates, and lead telluriums,which may be used alone or in combination of two or more. It isdesirable not to contain lead from the environmental viewpoint.

The glass frit may have a softening point of 400° C. to 600° C. Withinthe above range, it is advantageous for obtaining a dense conductivefilm having sufficient adhesive strength by virtue of no residual carbonand easy sintering. However, if the softening point is lower than theabove range, the removal of the binder may be difficult because thesintering of the glass frit starts before the resin component in thepaste evaporates. As a result, carbon may remain upon sintering, andsuch defects as peeling of the conductive film may occur. In addition,if the softening point is higher than the above range, it may bedifficult to obtain a dense conductive film having sufficient adhesivestrength.

The softening point can be calculated, for example, from the startingpoint of the second endothermic peak in the DTA curve obtained using athermogravimetric analyzer.

Organic Vehicle

The organic vehicle may comprise an organic solvent, a binder resin, andan additive.

The organic solvent is not particularly limited. For example, terpineol,butyl carbitol, butyl carbitol acetate, 2,2,4-trimethyl-1,3-pentanediolisobutyrate (Texanol™) or the like may be used alone or in combinationof two or more.

A cellulose resin, an epoxy resin, an acrylic resin, a polyester resin,a polyimide resin, a polyurethane resin, a phenoxy resin, a siliconeresin, or the like may be used alone or in combination of two or more asthe binder resin.

Specifically, a cellulose-based resin such as ethyl hydroxyethylcellulose (EHEC) and ethyl cellulose (EC) may be used as the binderresin.

Since ethyl hydroxyethyl cellulose among the above has goodcompatibility with an organic solvent, it is easily dissolved in anorganic solvent and can reduce the rate of change in viscosity of aconductive paste over time. However, ethyl hydroxyethyl cellulose is notgood in compatibility with some linear chain glycol ester organicsolvents (e.g., alkylene glycol diacetate, alkylene glycol dipropionate,and the like). In such event, one or more organic solvents having acyclic skeleton (e.g., cyclohexyl propionate,2-cyclohexyl-4-methyl-1,3-dioxane, dihydroterfinyl acetate, isobornylacetate, and the like) may be mixed to improve the compatibility.

Meanwhile, since ethyl cellulose has low solubility in organic solventsand the change in viscosity upon the preparation of a paste is large, itis desirable to use it in combination with ethyl hydroxyethyl cellulose.If the binder resin comprises ethyl hydroxyethyl cellulose and ethylcellulose together, it is preferable from the viewpoint of the rate ofchange in viscosity over time that the weight ratio of ethylhydroxyethyl cellulose to ethyl cellulose (i.e., EHEC/EC) is 1 or more.

In addition, if ethyl hydroxyethyl cellulose is used as the binderresin, the content of the ethoxy group in ethyl hydroxyethyl celluloseis more than 56% by weight, which is advantageous for preventing highviscosity and solubility reduction by virtue of sufficient compatibilitywith an organic solvent. The content of an ethoxy group may be measured,for example, by a gas chromatography method as provided in ASTMD4794-94. In this method, the hydroxyethoxy group (—OC₂H₄OH) and theethoxy group (—OC₂H₅) substituted to cellulose are measured withoutdistinction from each other. Thus, the term “content of an ethoxy group”in this specification may refer to the total content (% by weight) ofthe hydroxyethoxy group and the ethoxy group in ethyl hydroxyethylcellulose.

The content of the binder resin may be 0.4 to 3.0% by weight based onthe total weight of the conductive paste. Within the above contentrange, the strength and adhesion of the coating film are enhanced,whereby the binder may be readily removed.

In addition, the organic vehicle may further comprise an additive. Forexample, it may comprise a plasticizer, a metal oxide, a dispersant, aviscosity modifier, or the like.

Examples of the plasticizer include rosin ester, polyvinyl butyral,alkyl phthalate, and the like.

Specifically, the organic vehicle comprises a binder resin, an additive,and an organic solvent. It may comprise them in an amount of 0.4 to 3.0%by weight, 0.1 to 3.0% by weight, and 4.0 to 18.5% by weight,respectively, based on the total weight of the conductive paste.

Properties of the Paste

It is preferable that the conductive paste has an appropriate viscosity.If the viscosity is too low, the flowability increases and the electrodewidth becomes wide, which may lower the photovoltaic conversionefficiency. On the other hand, if the viscosity is too high, thecontinuous printability is deteriorated during printing due to cloggingof the screen, and it is difficult to obtain a uniform electrode patterndue to disconnection and step difference, which may increase theelectrode resistance.

In addition, it is preferable that the paste has a ratio of theviscosity under low-speed agitation conditions (e.g., 10 rpm) to theviscosity under high-speed agitation conditions (e.g., 100 rpm) in asuitable range (e.g., 3 to 6). If the ratio of viscosities is too low, adisconnection may occur due to a problem in continuous printability. Onthe other hand, if the ratio of viscosities is too high, there is noproblem in continuous printability, but steps may be formed.

According to the present invention, two or more types of sphericalsilver powders having different characteristics are mixed, whereby it ispossible to improve the characteristics of the product while thedisadvantages of each type of powders are minimized and the advantagesthereof are maximized. In addition, according to the present invention,the particle size distribution of the mixed silver powders and theparticle size and the specific gravity of the primary particles arecomprehensively controlled, so that it is possible to simultaneouslyachieve a highly densified conductor pattern, a precise line pattern,and suppression of coagulation over time.

Accordingly, the mixed silver powders are applied to a conductive pasteto increase the dispersibility and fluidity to enhance the conductivity,thereby keeping the electrode resistance low, maximizing the batteryefficiency, and securing the long-term product reliability. In addition,the process for preparing the mixed silver powders is simple, and theyhave excellent processability when applied to a conductive paste. Thus,it is possible to enhance the production efficiency.

Mode for the Invention

The present invention will be described in more detail with reference tothe following examples. However, these examples are provided toillustrate the present invention, and the scope of the examples is notlimited thereto only.

PREPARATION EXAMPLE 1 Silver Powder A1

25 kg of an aqueous ammonia solution of 25% by weight was added to 53 kgof an aqueous silver nitrate solution containing 16 kg of silver toprepare an aqueous silver solution. 0.3 kg of sodium alginate wasdissolved in 66 kg of water, followed by adjusting the pH to 10.5 withammonia and mixing 16 kg of hydrazine hydrate, to obtain an aqueousreducing agent solution. The temperature of the aqueous reducing agentsolution was maintained at 25° C. using a thermostat, and the aqueoussilver solution was uniformly added thereto over 6 hours to precipitatesilver. The precipitated silver was washed, filtered, dried, pulverizedusing a jet mill, and classified to obtain spherical silver powder A1.The silver powder A1 had a D₅₀ of 1.15 μm, a D₅₀/D_(SEM) of 1.01, a(D₉₀−D₁₀)/D₅₀ of 0.99, and a true specific gravity of 10.28.

PREPARATION EXAMPLE 2 Silver Powder A2

The procedure of Preparation Example 1 was repeated except that theaddition of the aqueous silver solution was carried out over 10 hours toobtain spherical silver powder A2. The silver powder A2 had a D₅₀ of1.58 μm, a D₅₀/D_(SEM) of 1.01, a (D₉₀−D₁₀)/D₅₀ of 0.99, and a truespecific gravity of 10.31.

PREPARATION EXAMPLE 3 Silver Powder A3

The procedure of Preparation Example 1 was repeated except that theaddition of the aqueous silver solution was carried out over 12 hours toobtain spherical silver powder A3. The silver powder A3 had a D₅₀ of1.83 μm, a D₅₀/D_(SEM) of 1.02, a (D₉₀−D₁₀)/D₅₀ of 1.04, and a truespecific gravity of 10.31.

PREPARATION EXAMPLE 4 Silver Powder B1

510 kg of an aqueous silver nitrate solution containing 16 kg of silverwas heated to 65° C. Added thereto was 56 kg of an aqueous ammoniasolution of 28% by weight to prepare a silver-ammonia complex solution.An aqueous solution obtained by diluting 53 kg of an aqueous solution offormaldehyde (formalin) of 37% by weight in 135 kg of water was added tothe silver-ammonia complex solution over 10 seconds to obtain a silverslurry. The silver slurry was washed with water and ethanol, filtered,dried, pulverized using a jet mill, and classified to obtain sphericalsilver powder B1. The silver powder B1 had a D₅₀ of 1.97 μm, aD₅₀/D_(SEM) of 1.46, a (D₉₀−D₁₀)/D₅₀ of 1.30, and a true specificgravity of 9.68.

PREPARATION EXAMPLE 5 Silver Powder B2

The procedure of Preparation Example 4 was repeated except that 510 kgof an aqueous silver nitrate solution containing 16 kg of silver washeated to 45° C. and that 48 kg of an aqueous ammonia solution of 28% byweight was added thereto to obtain spherical silver powder B2. Thesilver powder B2 had a D₅₀ of 1.76 μm, a D₅₀/D_(SEM) of 1.43, a(D₉₀−D₁₀)/D₅₀ of 1.37, and a true specific gravity of 9.53.

EXAMPLE 1 Preparation of Silver Powder C1

16 kg of silver powder A1 and 4.0 kg of silver powder B1 were put intoan intensive mixer, and 21 g of stearic acid (C18) and 9 g of palmiticacid (C16) were added for surface treatment. The temperature wasmaintained at 70° C. using a thermostat, and stirring was performed at1,500 rpm for 180 minutes to obtain silver powder C1.

EXAMPLE 2 Preparation of Silver Powder C2

10 kg of silver powder A2 and 10 kg of silver powder B2 were put into anintensive mixer, and 18 g of stearic acid (C18) and 42 g of palmiticacid (C16) were added for surface treatment. The temperature wasmaintained at 70° C. using a thermostat, and stirring was performed at1,500 rpm for 180 minutes to obtain silver powder C2.

EXAMPLE 3 Preparation of Silver Powder C3

13.2 kg of silver powder A3 and 6.8 kg of silver powder B2 were put intoan intensive mixer, and 50 g of stearic acid (C18) and 50 g of palmiticacid (C16) were added for surface treatment. The temperature wasmaintained at 70° C. using a thermostat, and stirring was performed at1,500 rpm for 180 minutes to obtain silver powder C3.

EXAMPLE 4 Preparation of Silver Powder C4

16 kg of silver powder A2 and 4.0 kg of silver powder B1 were put intoan intensive mixer, and 15 g of stearic acid (C18) and 15 g of palmiticacid (C16) were added for surface treatment. The temperature wasmaintained at 70° C. using a thermostat, and stirring was performed at1,500 rpm for 180 minutes to obtain silver powder C4.

EXAMPLE 5 Preparation of Silver Powder C5

10.0 kg of silver powder A1 and 10.0 kg of silver powder B1 were putinto an intensive mixer, and 42 g of stearic acid (C18) and 18 g ofpalmitic acid (C16) were added for surface treatment. The temperaturewas maintained at 70° C. using a thermostat, and stirring was performedat 1,500 rpm for 180 minutes to obtain silver powder C5.

EXAMPLE 6 Preparation of Silver Powder C6

13.2 kg of silver powder A2 and 6.8 kg of silver powder B2 were put intoan intensive mixer, and 70 g of stearic acid (C18) and 30 g of palmiticacid (C16) were added for surface treatment. The temperature wasmaintained at 70° C. using a thermostat, and stirring was performed at1,500 rpm for 180 minutes to obtain silver powder C6.

COMPARATIVE EXAMPLE 1 Preparation of Silver Powder D

16.0 kg of silver powder A2 was put into an intensive mixer, and 16.8 gof stearic acid (C18) and 7.2 g of palmitic acid (C16) were added forsurface treatment. The temperature was maintained at 70° C. using athermostat, and stirring was performed at 1,450 rpm for 180 minutes toobtain silver powder D.

COMPARATIVE EXAMPLE 2 Preparation of Silver Powder E

16.0 kg of silver powder B1 was put into an intensive mixer, and 7.2 gof stearic acid (C18) and 16.8 g of palmitic acid (C16) were added forsurface treatment. The temperature was maintained at 70° C. using athermostat, and stirring was performed at 1,450 rpm for 180 minutes toobtain silver powder E.

COMPARATIVE EXAMPLE 3 Preparation of Silver Powder F

10.0 kg of silver powder A1 and 10.0 kg of silver powder B1 were putinto an intensive mixer, the temperature was maintained at 70° C. usinga thermostat, and stirring was performed at 1,500 rpm for 120 minutes toobtain silver powder F.

COMPARATIVE EXAMPLE 4 Preparation of Silver Powder G

16.0 kg of silver powder A1 and 4.0 kg of silver powder B1 were put intoan intensive mixer, and 60 g of palmitic acid (C16) was added forsurface treatment. The temperature was maintained at 65° C. using athermostat, and stirring was performed at 1,500 rpm for 180 minutes toobtain silver powder G.

COMPARATIVE EXAMPLE 5 Preparation of Silver Powder H

16.0 kg of silver powder A2 and 4.0 kg of silver powder B1 were put intoan intensive mixer, and 60 g of stearic acid (C18) was added for surfacetreatment. The temperature was maintained at 75° C. using a thermostat,and stirring was performed at 1,500 rpm for 180 minutes to obtain silverpowder H.

The compositions of the silver powders prepared in the above Examplesand Comparative Examples are summarized in Tables 1 and 2 below.

TEST EXAMPLE 1 Analysis of Silver Powder

The silver powder was measured for the particle size distribution bylaser diffraction using Analysette22 of Fritsch. The silver powder wasdegassed at 100° C. for 60 minutes and measured for the specific surfacearea (BET) using Belsorp-miniII of MicrotracBEL. 30 g of the silverpowder placed in a 10-ml container and measured for the true specificgravity using Accupyc II of Micromeritics. 15 g of the silver powder wasplaced in a 20-ml test tube and subjected to 2,000 taps with a 20 mmdrop using Autotap of Quantachrome for the measurement of the tapdensity (TD).

The results are shown in Tables 1 and 2 below.

In addition, FIG. 1 is an electron microscope image of a cross-sectionof the silver powder A3 prepared in Preparation Example 3 cut with afocused ion beam (FIB). FIGS. 2 and 3 are an electron microscope imageof a cross-section of the silver powder B2 prepared in PreparationExample 5 cut with an FIB and one for measuring the area of poresobtained by image processing. In FIGS. 1 to 3, it was confirmed that theporosity of the silver powder B2 was larger than that of the silverpowder A3.

TEST EXAMPLE 2 GC-MS and SDT

Pyrolysis gas chromatography mass spectrometry (pyrolysis GC-MS) for theorganic substances coated on the surface of the silver powder wascarried out by Pyrolyzer GC-MS of Agilent Technologies. In addition,simultaneous DSC and TGA (SDT) was carried out by SDT-Q600 of TA Koreaat room temperature from 600° C. to 10° C./min.

The results are shown in Tables 1 and 2 below.

In addition, FIG. 4 shows the result of GC-MS of the silver powder C2prepared in Example 2. FIGS. 5 and 6 show the results of GC-MS and SDTof the silver powder C5 prepared in Example 5. In FIGS. 5 and 6, it wasconfirmed that two types of fatty acids were adsorbed on the surfaces ofthe silver powders. In addition, in FIG. 6, it was confirmed that thesilver powders on which the two types of fatty acids had been adsorbedhad a weight increase peak and an exothermic peak at a temperature of200° C. to 300° C.

TEST EXAMPLE 3 Evaluation of Silver Powder

90 parts by weight of the silver powder, 2 parts by weight ofPbO-TeO-based glass frit, and 8 parts by weight of an organic vehicle(including an EC-based binder resin, additives, and an organic solvent)were mixed and stirred by a planetary mixer and further dispersed usinga 3-roll mill, followed by removing dust and impurities throughfiltration, to prepare a conductive paste.

The viscosity of the conductive paste was measured at 25° C. under theconditions of 10 rpm and 100 rpm using a Brookfield HB viscometer.

Thereafter, the conductive paste was screen printed on a P-typecrystalline silicon wafer through a screen (3 bus bars and 158 fingerpatterns), dried, and sintered to prepare an electrode.

In order to evaluate the continuous printability at the time ofpreparing the electrode, EL disconnection was measured using K3300EPLIsolar cell imaging test system equipment of Mcscience while theconductive paste was screen printed onto 1,000 wafers. As a result, thecontinuous printability was evaluated as “very good” when the ELdisconnection observed was 1 or less, “good” when 2 or 3, “normal” when4 or 5, and “poor” when 6 or more.

In addition, the resistance of the electrode was measured as theresistance of the finger electrode portion of the sintered wafer using asurface resistance meter of CMT-100S model of AIT.

In addition, the photovoltaic conversion efficiency (%) of a solar cellusing the electrode was measured using CT-801 of Pasan.

The results are shown in Tables 1 and 2 below.

TABLE 1 Item Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Silver powder C1 C2 C3C4 C5 C6 Mixing ratio Silver powder A 80 50 66 80 50 66 (part by weight)Silver powder B 20 50 34 20 50 34 D_(SEM) 1.18 1.27 1.37 1.48 1.25 1.33D₁₀ 0.491 0.758 0.744 0.977 0.602 0.749 D₅₀ 1.216 1.613 1.542 1.6521.278 1.488 D₉₀ 2.475 2.632 2.477 2.732 2.226 2.391 D₅₀/D_(SEM) 1.031.27 1.13 1.12 1.02 1.12 [D₉₀ − D₁₀]/D₅₀ 1.63 1.16 1.12 1.06 1.27 1.10Specific surface area (BET, m²/g) 0.501 0.322 0.366 0.415 0.418 0.374True specific gravity 10.08 9.90 9.78 10.08 9.9 9.78 Tap density (TD,g/cc) 4.35 4.43 4.95 5.23 4.7 4.68 Fatty acid C16 fatty acid 30 70 50 5030 30 (part by weight) C18 fatty acid 70 30 50 50 70 70 Coated amount offatty acid (wt. %) 0.15 0.3 0.5 0.15 0.3 0.5 Viscosity (100 rpm, kcPs)50 51 60 48 53 58 Ratio of viscosities 4.26 3.95 3.33 4.69 4.16 3.53 (10rpm/100 rpm) Continuous printability Good Very Very Very Very Good goodgood good good Electrode resistance 50.61 51.62 57.07 53.89 49.71 56.97(mΩ, finger portion) Photovoltaic conversion efficiency 18.329 18.44518.399 18.386 18.591 18.412 of solar cell (%)

TABLE 2 Item C. Ex. 1 C. Ex. 2 C. Ex. 3 C. Ex. 4 C. Ex. 5 Silver powderD E F G H Mixing ratio Silver powder A 100 0 50 80 80 (part by weight)Silver powder B 0 100 50 20 20 D_(SEM) 1.51 1.34 1.24 1.21 1.51 D₁₀1.002 0.875 0.631 0.523 0.991 D₅₀ 1.576 1.956 1.368 1.299 1.699 D₉₀2.562 3.413 2.524 2.681 3.001 D₅₀/D_(SEM) 1.04 1.46 1.10 1.07 1.13 [D₉₀− D₁₀]/D₅₀ 0.99 1.30 1.38 1.66 1.18 Specific surface area (BET, m²/g)0.428 0.361 0.452 0.487 0.304 True specific gravity 10.2 9.6 9.9 10.0810.08 Tap density (TD, g/cc) 6.18 4.59 4.01 4.67 4.78 Fatty acid C16fatty acid 30 70 0 100 0 (part by weight) C18 fatty acid 70 30 0 0 100Coated amount of fatty acid (wt. %) 0.15 0.15 0 0.3 0.3 Viscosity (100rpm, kcPs) 42 63 — 51 46 Ratio of viscosities 4.53 2.71 — 4.04 3.93 (10rpm/100 rpm) Continuous printability Normal Poor — Normal Poor Electroderesistance 49.66 58.66 — 54.26 53.14 (mΩ, finger portion) Photovoltaicconversion efficiency 18.154 18.338 — 18.212 18.115 of solar cell (%)

As shown in Tables 1 and 2 above, two types of spherical silver powdershaving different characteristics were mixed and two surface treatmentagents were coated on the surfaces in Examples 1 to 6, so that theviscosity, viscosity ratio, continuous printability, and electroderesistance of the conductive paste and the photovoltaic conversionefficiency of the solar cell were excellent.

In contrast, only one type of silver powder was used in ComparativeExamples 1 and 2, and only one type of a surface treatment agent wasabsorbed in Comparative Examples 4 and 5. As a result, at least one ofthe viscosity, viscosity ratio, continuous printability, and electroderesistance of the conductive paste and the photovoltaic conversionefficiency of the solar cell was poor.

Meanwhile, a surface treatment agent was not absorbed onto the silverpowder in Comparative Example 3. As a result, the silver powder was notdispersed in the organic vehicle and was not coated to the wafer duringscreen printing, so that the evaluation could not be made.

1. A mixed powder comprising two or more spherical silver powders,wherein the two or more spherical silver powders having a differentparticle size distributions from each other, wherein the sphericalsilver powders comprise two or more surface treatment agents on theirsurfaces, and when the cumulative 10%, 50%, and 90% particle sizes byvolume in the particle size distribution of the mixed silver powdersobtained by laser diffraction are referred to as D₁₀, D₅₀, and D₉₀,respectively, and the average particle size of the primary particlesobtained by image analysis of a scanning electron microscope is referredto as D_(SEM), the D₅₀ is 0.5 to 2.5 μm, the D₅₀/D_(SEM) is 1.0 to 1.5,the (D₉₀−D₁₀)/D₅₀ is 1.0 to 2.0, and the true specific gravity is 9.4 to10.4.
 2. The mixed powder of claim 1, which have a specific surface areaof 0.1 to 2.0 m²/g and a tap density of 2.0 to 7.5 g/cc.
 3. The mixedpowder of claim 1, which comprises silver powder A; and silver powder Bhaving a particle size distribution different from that of the silverpowder A, wherein the silver powder A has a D₅₀ of 0.5 to 2.5 μm, aD₅₀/D_(SEM) of 1.0 to 1.2, a (D₉₀−D₁₀)/D₅₀ of 0.9 to 1.2, and a truespecific gravity of 10.0 to 10.4, and the silver powder B has a D₅₀ of0.7 to 3.5 μm, a D₅₀/D_(SEM) of 1.0 to 1.5, a (D₉₀−D₁₀)/D₅₀ of 1.0 to2.0, and a true specific gravity of 9.3 to 10.0.
 4. The mixed powder ofclaim 3, which comprise the silver powder A and the silver powder B in aweight ratio of 90:10 to 10:90.
 5. The mixed powder of claim 1, whereinthe surface treatment agents comprise two or more types selected fromthe group consisting of a C16 fatty acid, a C18 fatty acid, a derivativethereof, and a salt thereof.
 6. The mixed powder of claim 5, wherein thesurface treatment agents comprise the C16 fatty acid and the C18 fattyacid at a weight ratio of 20:80 to 80:20.
 7. A mixed powder comprisingtwo or more silver powders, wherein the two or more silver powderscomprise two or more surface treatment agents on their surfaces and havea weight increase peak at a temperature of 200° C. to 300° C. inthermogravimetric analysis (TGA) under an elevated temperature conditionof 10° C./min.
 8. A conductive paste, which comprises the mixed powderof claim
 1. 9. The conductive paste of claim 8, which comprises 80 to95% by weight of the mixed powder; 0.5 to 3.0% by weight of glass frit;and 4.5 to 19.5% by weight of an organic vehicle.
 10. The conductivepaste of claim 9, wherein the organic vehicle comprises a binder resin,an additive, and an organic solvent in an amount of 0.4 to 3.0% byweight, 0.1 to 3.0% by weight, and 4.0 to 18.5% by weight, respectively,based on the total weight of the conductive paste.
 11. A solar cell,which comprises an electrode formed from the conductive paste of claim8.
 12. A conductive paste comprising the mixed powder of claim
 2. 13. Aconductive paste comprising the mixed powder of claim
 3. 14. Aconductive paste comprising the mixed powder of claim
 4. 15. Aconductive paste comprising the mixed powder of claim
 5. 16. Aconductive paste comprising the mixed powder of claim
 6. 17. Aconductive paste comprising the mixed powder of claim 7.